Conventional alternating current generators or alternators may include a motor winding (the field winding), three-phase stator windings (i.e. Delta or Wye configured), and a three-phase full wave controlled rectifier bridge. In order to convert the alternating current (AC) generated in the stator windings to direct current (DC) a rectifier bridge is connected to the stator windings. The rotor of the alternator is connected to a vehicle engine which turns the rotor that holds the field winding. The rotation of the rotor and thus the field winding by the vehicle engine causes AC power to be induced in the stator windings. The power generated in the stator windings is typically three-phase power. The voltage generated in each of the phases is delivered to the full wave rectifier bridge where it is converted into DC power for delivery to the vehicle load (i.e. the battery and vehicle electrical system). A battery is connected in parallel with the outputs of the full wave rectifier bridge for delivering adequate power to the load when the field winding is not rotating or when the field winding is rotating too slowly to result in a voltage equal to the battery voltage. However, when the rotor and field winding rotate at a sufficient speed, a voltage is generated across the battery terminal that is greater than the battery voltage and the battery is recharged. In the case where the vehicle engine is idling, such that the field winding rotates at a less than sufficient speed, the output of the alternator may not be adequate to supply all the power required by the load. When this occurs, the alternator is no longer regulating the system voltage. The battery is being discharged as it attempts to supplement the alternator output to meet the power demand at the load. If this condition remains over an extended period of time, the battery will become completely discharged.
The most common way to control the output of the alternator during engine idling and providing extra power for delivery to the vehicle load is to increase the rotor field flux. This may be accomplished by increasing the current through the field winding. If a higher current is delivered through the field winding, a greater voltage will be induced in the stator windings and a higher output from the alternator will result as the engine idles and the rotor rotates at a slow speed. However, the amount of current that may be delivered to the field winding is limited by overheating concerns, as too much current flow through the field winding may cause the alternator to overheat.
Other prior art methods for obtaining an increased output from the alternator during engine idle is to maximize the power angle. The power angle is defined as the phase difference between the back EMF generated in the stator windings and the phase voltage output from the stator windings.
In a passive diode bridge (i.e., rectifier bridge where only diodes are used), the phase current and the phase voltage are forced to be in the same phase relationship. When the phase voltage and phase current are forced to be in the same phase relationship, it is not possible to achieve an optimal power angle such that the back EMF and the phase voltage are orthogonal. However, if the passive bridge is replaced by a controlled or active transistor bridge (i.e., a rectifier bridge where the diodes are replaced by transistor switches), the phase voltage may be allowed to lag the phase current. When the phase voltage is allowed to lag the phase current, the phase angle between the back EMF and the phase voltage may approach the optimal 90° mark. It is known that the power output of an alternator at idle speeds can be increased by 45–50% by optimizing the power angle towards 90° by advancing the phase angle of the phase voltage.
In a controlled rectifier bridge, the angle of the phase voltage can be controlled by turning the transistor switches on and off at selected times. If the angle of the back EMF is known, the angle of the phase voltage may be adjusted by the switches in the controlled rectifier bridge and a more optimal power angle may be introduced. Unfortunately, it is difficult to obtain a direct reading from the back EMF generated in the stator windings. Without a machine reference from the back EMF, the optimal phase angle for the phase voltages cannot be determined. Therefore, some method must be used to obtain an indication of phase angle of the back EMF before the phase voltage is shifted by the controlled rectifier bridge.
One prior art method utilizes a rotor position sensor to provide a back EMF reference. Another method requires a current sensor to determine the phase current. An adjustment of the angle between the phase current and the phase voltage affects the power angle. Still other methods monitor the third harmonic voltage at the neutral point of a Wye-wound alternator and switch the transistors based on voltage levels measured.
In yet another method, the voltage across the low side transistor is monitored to determine when the current reverses through the low side transistor. By knowing the speed of the alternator, the current reversal of the high side transistor may be inferred.
While the prior art methods achieve their intended purpose, there still is a need for new and improved systems and methods for controlling the voltage output of an alternator at varying engine speeds and especially at lower engine speeds.